The conventional form of meter prover utilised the passage of a sphere along an accurately dimensioned pipe between fixed detectors to displace a known volume of fluid. The volume of fluid displaced by the prover passes in series through the meter to be proved and the number of pulses generated in the meter during the passage of the sphere between the detectors is counted to enable determination of the k factor. To achieve the necessary high accuracy, a large displaced volume is essential and provers of 20 meters in length were not uncommon. Provers of this size are clearly impractical on oil rigs and other confined spaces and efforts have therefore been made to develop so called compact provers. By increasing the inherent accuracy of the proving operation with the use of electrical pulse handling techniques, it has been possible to reduce the displaced volume necessary for the desired 0.02% accuracy and meter provers have been produced in the form of a piston and cylinder having a stroke in the order of one meter.
With meter provers of the piston type, the difficulty is encountered of how to restore the piston to its start position after the end of a proving stroke. Various mechanisms have been suggested for this purpose including cable and winch arrangements (such as for example shown in FR No. 2 471 590) or separate retraction pistons operating on compressed gas (see for example GB No. 2 023 295). Any such additional retraction mechanism requires to be carefully controlled so as to offer no impedance whatever to the proving stroke but to restore the piston fully and reliably to its start position in time for the next proving stroke. These meter provers have therefore been relatively expensive. Moreover, the retraction mechanism represents a further source of possible malfunction or breakdown.
Other meter provers restore the piston by reversing the flow through the cylinder, thus offering the possibility of proving in both directions movement of the piston. Provers of this sort are referred to as bidirectional in contrast to the unidirectional provers with separate retraction mechanisms described above. If a bidirectional prover is to be connected in line with a working meter, it is necessary to provide a four way valve such that flow can be directed in opposite senses through the prover and in a by-pass mode around the prover. Care must be taken to monitor any possible leakage in such four way valves using, for example, block and bleed techniques. Four way valves with block and bleed facilities are, however, relatively expensive. There is a further difficulty associated with bidirectional provers, namely that the detection, control and processing systems have to be capable of dealing with proving strokes in two opposite directions. This invariably adds to the complexity and cost of the prover.